Double aromaticity arising from σ- and π-rings

Abstract

Aromaticity has been a central concept in chemistry since the discovery of benzene in the 19th century and has impacted the science of delocalized π-electron systems. The aromaticity of conventional aromatic compounds usually originates from electron delocalization through a single ring that consists of π-symmetric orbitals. Although double aromaticity, i.e. aromaticity composed of two circularly delocalized orbitals, has been theoretically predicted for over 20 years, the double aromaticity of a bench-stable compound is not well explored by experiment. Here we report the synthesis and isolation of the dication of hexakis (phenylselenyl)benzene, as well as its double aromaticity based on structural, energetic, and magnetic criteria. In this dication, cyclic σ-symmetric and π-symmetric delocalized orbitals are formally occupied by ten and six electrons, respectively, and the aromaticity thus follows the 4n + 2 (n = 1, 2,…) electron Hückel rule, regardless of the σ-orbital or π-orbital symmetry.

“Bottled” spiro-doubly aromatic trinuclear [Pd2Ru]+complexes

“…The only bonding combination that satisfies ON threshold and does not break the quasi-symmetry of the compound is the inner triangle-based 3c-2e σ-bond. Therefore, it complies with classical 4n+2 rule which was extended from π-aromatics to σ-aromatics 5,86,87 . According to this rule, aromatic compounds possess 4n+2 electrons participating in aromaticity where n is an integer.…”

“…[42][43][44][45] However, to the best of our knowledge, none were obtained as stable phases in the bulk except Saito's one. 5 Thus, reported here liganded triangular TM-based complex is the second "bottled" example of double aromaticity which is another milestone towards expansion of aromaticity concept throughout the chemistry space. Moreover, this is the first reported system with Ru atom taking part in two types of aromaticity simultaneously.…”

“…They managed to synthesize a doubly aromatic compound [C 6 (SePh) 6 ] 2+ that possesses cyclic σ-symmetric and π-symmetric aromatic rings. 5 This milestone indicates that other types of aromaticity apart from the conventional π-type are not elusive. Obviously, even a hundred years later, the paradigm of aromaticity is undergoing alterations and expansions.…”

Towards understanding π-stacking interactions between non-aromatic rings

“…The -polar model (Hunter et al, 2001) of aromatic interactions has been refined over time, taking into consideration direct substituent interactions and solvation/desolvation effects (Hunter & Sanders, 1990;Janiak, 2000;Hunter et al, 2001;Salonen et al, 2011;Martinez & Iverson, 2012;Wheeler & Bloom, 2014;Carini et al, 2017). Face-to-face stacking is possible between electronrich aromatics and electron-depleted ones such as hexafluorobenzene (also an example of double aromaticity arising from -orbital and -orbital interactions; Furukawa et al, 2018) and is often referred to as an aromatic donor-acceptor interaction; some level of -orbital mixing occurs and the donor-acceptor term better describes a situation in which relatively electron-deficient and electron-rich aromatic molecules stack in an alternating fashion (Hunter & Sanders, 1990;Martinez & Iverson, 2012). These cases are, however, rare.…”

Searching for double σ- and π-aromaticity in borazine derivatives

“…1 Since then, many compounds exhibiting double aromaticity have been predicted theoretically, [2][3][4][5][6][7][8][9][10][11] with some even being experimentally characterized. [12][13][14][15][16][17] For instance, in 1988, Martin and Sagl 15 reported the synthesis of the hexaiodobenzene dication, C 6 I 6 2+ (2, Scheme 1), a double-aromatic compound with two concentric delocalization circuits, consisting of 6p electrons delocalized around the C 6 ring, and 10s electrons delocalized around the I 6 ring. This nding paved the way to synthesize and characterize, by X-ray diffraction, the rst double-aromatic "bench-stable" molecule, the hexakis(phenylselenyl)benzene dication 16 (3, Scheme 1).…”